Hardened polyvinyl alcohol silver halide photographic coatings



Patented Sept. 13, 1949 HARDENED POLYVINYL ALCOHOL SILVER HALIDE PHOTOGRAPHIC COATINGS Wesley G. Lowe, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application August 13, 1947, Serial N0. 768,477

7 Claims.

This invention relates to the hardening of polyvinyl alcohol coatings by means of a complex made up of urea, formaldehyde and polyvinyl alcohol. This invention relates more particularly to the hardening therewith of coatings of photographic emulsions containing polyvinyl alcohol as the carrier therein.

This application 'is a continuation in part of my application Serial No. 497,391yfi1ed August 4, 1943, now abandoned.

In the manufacture of photographic emulsions in which gelatinis employed as the base, it is customary in cases where the emulsion is to be subjected to hot ferrotyping or to aqueous processing solutions under warm conditions to employ hardener'for the gelatin. By this hardening the sticking of the emulsion to the hot ferrotyping roll is reduced or substantially eliminated and also processingof the emulsion is facilitated.

Recently the use of polyvinyl alcohol has been suggested for photographic purposes, particularly as the carrier for the light-sensitive material. For instance, silver halide emulsions have been referred to in which polyvinyl alcohol has been employed'as the protective colloid therefor. Also polyvinyl alcohol has been used for coating purposes in many instances. 'Although a number of hardeners have been suggested for use with polyvinyl alcohol, various disadvantages have been encountered in that use. For instance, some hardeners, while giving a suitable hardening action, have 'exhibiteda desensitizing or fogging action upon the photographic emulsion. Other materials which do notcause desensitization have been found not to be sufllciently effective to render the polyvinyl alcohol emulsion resistant to the hot rollers commonly employed in imparting a glazed finish to photographic prints. Some of the hardeners suggested have been of a, diffusible nature so that when the polyvinyl alcohol coating was applied to paper the hardener would wander into the paper so that its influence was lost. This action could be counteracted by using either a hardener having a rapid hardening action or by using a hardener that was virtually non-diffusible,

One object of my invention 'is to provide a hardener for polyvinyl alcohol coatings of a nondiffusible nature. Another object of my invention is to provide a hardener for hardening polyvinyl alcohol coatings which does not desensitize or fog photographic emulsions but yet is sufficiently effective to render the polyvinyl alcohol layer resistant to hot rollers. Other objects will appear herein.

I have found thathardened polyvinyl alcohol coatings may be prepared by mixing with an aqueous solution of polyvinyl alcohol a complex of urea, formaldehyde andpolyvinyl alcohol, coating out the solution onto a surface and drying the coating. I have found that the action on the polyvinyl alcohol layer is not exerted until the drying operation takes place so that no increase in the viscosity of the solution used for coating is experienced. I have also found that this method is especially useful in the preparation of hardened polyvinyl alcohol photographic emulsion layers without desensitization of the photosensitive element occurring; The melting points of polyvinyl alcohol coatings prepared in accordance with my invention are raised to high temperatures in contrast to the coatings obtained from polyvinyl alcohol solutions without the use of hardener. v

My invention broadly comprises the incorporation of a proportion of urea-formaldehydepolyvinyl alcohol complex into the polyvinyl coating composition, followed by coating out of the polyvinyl coating composition onto a support and drying of the coating formed. The hardener which is added is measured in terms of formaldehyde present therein and the proportion of hardener used is within the range of 1-27 based on the poly-vinyl alcohol which is present. For instance, if a batch of hardener contains 5 parts of formaldehyde and is added to an aqueous solution of polyvinyl alcohol the final solution containing 50 parts of polyvinyl alcohol, then the hardener is regarded as being present in a 10% proportion, being the proportion of formaldehyde present based on the polyvinyl alcohol in the coating composition (which includes also the polyvinyl alcohol in the hardener) My hardener is particularly useful for forming hardened polyvinyl alcohol-silver-halide photographic emulsion layers in view of the fact that the sensitizedemulsion itself will not stand prolonged heating. The hardener complex is prepared by heating the formaldehyde, urea, and polyvinyl alcohol together and then adding to the aqueous polyvinyl alcohol in the desired amount.

The hardener in accordance with my invention may be prepared by the simultaneous reaction of the three ingredients. The reaction may be carried out at the pH obtained by mixing these materials together, or it mayjbe carried out at a pH above '7, such asimparted'by the use of a small amount of ammonia or astrong amine. The complex is formed byreacting together the urea,

formaldehyde and polyvinyl alcohol at an elevated temperature, such as between 50 and 100 C. and preferably between 70 and 90 C. The heating is only continued long enough to obtain some reaction of these ingredients and is carried out only for a sufdcient time that the materials remain water soluble. The resulting hardener should either be employed at once or kept at a pH of at least 7 or more to assure its keeping.

The hardener employed in accordance with my invention is prepared by reacting together 0.5-1.5 parts by weight of formaldehyde, 0.5-2 parts by weight of urea and 1 to 10 parts of polyvinyl alcohol. These materials are mixed together'and the amount of hardener added is figured in terms of the formaldehyde content of the composition. Very desirable proportions of these materials for use in preparing hardener are equal parts of urea and polyvinyl alcohol and an amount of formaldehyde of about 1% parts. The preparation of the urea-formaldehyde-polyvinyl alcohol complex which is suitable for use as a hardener in accordance with our invention is illustrated herein and is also described on page 2, lines 16 to 25, of British Patent 361,910,

The speed with which the polyvinyl alcohol coating composition is hardened depends upon several factors, one of the important ones being the proportion of hardener which is used. This proportion is based on the amount of formaldehyde present in the hardening agent, compared with the amount of polyvinyl alcohol which is present in the coating composition. It is desirable for rapid hardening that at least 4% of hardener be employed in the composition although even 1% is enough in some cases. Another factor which determines the speed of hardening is the pH of the coating solution. The more acid the coating solution, the more rapid is the hardening action effected, particularly down to a pH of 3-4. Another factor which influences the speed of hardening is the concentration of the polyvinyl alcohol in the coating solution used. For instance, a 10% solution of polyvinyl alcohol upon coating out hardens more quickly than a 2% solution. Another factor which influences the rate at which the hardening occurs is the rate of drying of the polyvinyl alcohol coating. I have found that polyvinyl alcohol coatings which have been dried slowly harden at a greater rate than polyvinyl alcohol coatings which have been subjected to rapid drying. Some coatings in accordance with my invention harden within a few hours. Other coatings in accordance with my invention may harden slowly due to one or several conditions employed in their preparation and days may be required to get a coating of good resistance to water or having a high melting point.

My hardening method is especially designed for use with polyvinyl alcohol-silver-halide photographic emulsions which have been characterized heretofore by difficulty in obtaining hardened coatings. sensitize-d photographic emulsions cannot be subjected to an elevated ternperature without affecting their sensitization and, therefore, any hardening method involving hardening thereof 'by elevated temperatures is detrimental. Also, hardening methods involving the action of many hardening agents may be out of the question due to their derogatory effect on sensitized photographic emulsions. By preparing a urea-formaldehyde-polyvinyl alcohol composition at an elevated temperature initially and adding this composition to polyvinyl alcohol pho- 4 tographic emulsions good hardening is obtained (the time of hardening depending upon conditions) without deleteriously affecting the emulsion. 7

The hardener or complex of urea, formaldehyde and polyvinyl alcohol may be prepared either with or withoutthe use of a base, such as ammonia in addition to the heating employed.

The complex which is prepared when ammonia is' used hardens the polyvinyl alcohol coating at either an acid or an alkaline pH, whereas the complex prepared at an acid pH, such as in the absence of ammonia or other base, will harden the polyvinyl alcohol coating only at a pH below 7. Polyvinyl alcohol coatings in which the hardener prepared atan alkaline pH is used may be "set upon coating by employing boric acid (or acidulated borax) in the coating solution and subsequently fuming the layer formed therewith with gaseous NHs as described and claimed in Lowe and Griflin application, Serial No. 501,990, filed September 11, 1943, now Patent No. 2,376,-

371, May 22, 1945. In this way the polyvinyl an cohol coating can be set and also upon drying and ageing a hardened coating will be obtained.

I have found that incorporation of 16% of hardener in the polyvinyl alcohol coating composition can generally be depended upon to give the maximum hardening'effect although in most cases lesser amounts are. satisfactory. In View of the fact, however, that this hardener has very slight desensitizing action on photographic emulsions, proportions thereof greater than 16% may be employed if desired by the individual operator.

Wherever the term polyvinyl'al-cohol is employed herein, it refers to a partially hydrolyzed polyvinyl ester, the major portion of which is polyvinyl alcohol. For instance, commercial polyvinyl alcohol is ordinarily prepared by hydrolyzing polyvinyl acetate down to a point where a small percentage of polyvinyl acetate remains, it being impractical to completely hydrolyze the polyvinyl acetate. .In some cases the amount of hydrolysis is less than in other cases but all compoun s designated as po yv yl alcohols contain more than 50% polyvinyl alcohol, and ordinarily contain 95 to 99% polyvinyl alcohol.

The following examples illustrate the preparation of hardeners and their use for hardening polyvinyl alcohol coating compositions in accordance with my invention:

Example 1.45 cc. of aqueous formaldehyde of 20% concentration, 6 grams of urea; 50 cc. of an aqueous solution of polyvinyl alcohol of 12.5% concentration and 0.6 cc. of concentrated am monium hydroxide were mixed together and heated at 90 C. for 30 minutes and the resulting solution was then adjusted to a pH of 8 and a volume of 90 cc. The solutionwas considered as a 10% solution of hardener (since it contains 10% formaldehyde). The resulting solution was found to be completely stable at room temperature. 7. Example 2.A 5% solution of freshly hydrolyzed polyvinyl acetate (polyvinyl alcohol) cone;

taining 26% polyvinyl acetate, the remainder being polyvinyl alcohol (the solvent being 75 water and 25% ethyl alcohol) was adjusted to a pH of" 3.5 with acetic acid. Tosamples of this solution were added varying amounts of urea-formal dehyde-polyvinyl alcohol complex hardener as prepared in accordance with Example l, the proportions being figured in accordancewith the amount of formaldehyde in the hardener and' based on the polyvinyl alcohol (the. entire hydro-,

Per Cent of Herdenerbalculated asthe Per Cent of Formaldehyde Based on Hardening 3 922 Total Polyvinyl Alcohol Present in the Behavior y g CoatingComposition V (Hmlrs) Example 3;--A photographic emulsion'essentially consisting of silver halide and as the vehicle therefor partially hydrolized polyvinyl acetate having a 33% polyvinyl acetate content, the remainder beingpolyvinyl alcohol, in suspension in azsolven't consisting :of 75% water and 25% ethyl alcohol/was adjustedto a pH of 4 with acetic acid. Samples of "the photographic emulsion were mixed with varying-amounts of the ureaformaldehyde-polyvinyl alcohol hardener and the various samples were'coated on paper. The percentage of hardener used was based on the percentage of formaldehydetherein to the total hydrolyzed polyvinyl acetate referred to herein as polyvinyl alcohol in the coating composition. The coatings were dried for sixteen hours and process-ed in the usual manner for processing photographic emulsions. "Such coatings where no hardener is used dissolve off in the wash water after fixing. The following table shows the hardening action obtained after sixteen hours drying'with the various percentages of hardener.

Per Cent of Hardener Fignredas Per Cent of. Formaldehyde Based on Total Polyvmyl Alcohol in the Emulsion Processing Behavior Image can he rubbed ofi.

Slight slipperiness on prolonged washing; image cannot be rubbed ofl.

Excellent hardening and not slip- "peryat all.

Excellent hardening.

12% polyvinyl alcohol (22 seconds viscosity) cc 50 20% formaldehyde cc.. 45 Urea g 12 28% ammonia cc 0.6

The mixture was heated for fifteen minutes at 90 C. It was then cooled and the pH was adjusted to 7.5.

Example 5.A polyvinyl alcohol-silver bromide emulsion was prepared containing silver halide corresponding to 100 g. of silver nitrate and 100 g. of polyvinyl alcohol. The volume of this emulsion was 3000 cc. To this emulsion was added 50 cc. of a 4% aqueous borax solution (the pH of the borax solution was adjusted to 5.5 with acetic acid before adding). 200 cc. of the hardener prepared as described in Example 4 was added and the emulsion was coated onto a surface, fumed 6. with gaseous ammonia to'set the same, and 'dried by allowing to stand at room temperature. After ten days the emulsion coating was found to be insoluble in water at C. The amount of hardener used in this example was 17.8% based on the formaldehyde content of the hardener and the total polyvinyl alcohol present both in the emulsion composition and the hardener. It was found that the hardener does not desensitize photographic emulsions regardless of the quantity used. thus distinguishing from the better known hardeners heretofore used for "polyvinyl alcohol coating. r

Example 6.-A hardener was prepared in the absence of ammonia (or other alkaline material) as follows: 100 cc. of 10% aqueous polyvinyl alcohol was mixed with cc. of 20% formaldehyde solution and 12 g. of urea. Due to the acidity of the formaldehyde solution it was found that the pH of the mixture was 4.3. The mixture was heated for thirty minutes at 70 C. during which time the viscosity of the solution rose to three times its original value. The solution was cooled and its pH was adjusted to '8. This hardener was added to various samples of a 5% solution of high viscosity polyvinyl alcohol in various proportions, coated out on glass plates, and dried at room temperature. The coating was aged for twenty-four hours. The conditions and results obtained were as follows:

The coatings which exhibited solubility after twenty-four hours aging but which contained hardener (the latter two) were allowed to stand for a longer period whereupon hardening of the layers occurred. When the hardener is used'in photographic emulsions coated onto paper, the hardening of the coatings is much slower than in coatings made on glass plates or on film. With 8-12% of hardener at a pH of 4 in polyvinyl alcohol photographic emulsions, used for coating paper, two to four weeks aging is usually desirable to reach a hardness sufficient for the paper to resist the melting action of hot ferrotyping.

The test of the effectiveness of a hardening agent for a partially hydrolyzed polyvinyl acetate or polyvinyl alcohol is the resistance of a coating thereof to hot water as compared with a check of a polyvinyl alcohol coating which has been hardened by some other means. It was observed that with polyvinyl alcohol coatings which had completed hardening in accordance with my invention the layer or coating remained in place and was resistant to softening when hot water was applied. The behavior of these coatings in the hot water test compared favorably with the best hardeners for polyvinyl alcohol coatings known at present. The time employed in preparing the hardener ordinarily should be at least fifteen minutes in order to obtain the desired complex. With temperatures in the lower portionrofthe range indicated, it may be desirable to -use longer times such as even as much as one-half hour to an hour to obtain the most desirable eifectiveness. Some cases, such as in the preparation of the compounds described in British Patent 361,910 two or three hours'may even be desirable to obtain the desired hardener. -1Iclaim z- W 1; A, olyvinyl alcohol-silver halide photographic emulsion adapted to give coatings resistant to the softening efiect of hot rolls comprising silver halide dispersed in a polyvinyl alcohol protective colloid containingtherein a hardener prepared by heating together at 50100 C. 0.5- 1.5 parts of formaldehyde, 0.5-2 parts of urea, and 1-10 parts of polyvinyl alcohol, the hard ener being present in the emulsion in the proportion of 1-27 parts of hardener based on the formaldehyde therein per 100 parts of polyvinyl alcohol.

2. A polyvinyl alcohol-silver halide photographic emulsion adapted to give coatings resistant to the softening eifect of hot rolls comprising silver halide dispersed in a polyvinyl alcohol protective colloid containing therein a hardener prepared by heating together at 50- 100 C. 1 /2 parts of formaldehyde, 1 part of urea, and 1 part of polyvinyl alcohol, the hardener being present in a proportion of 1-27 parts of hardener based on the formaldehyde therein per 100 parts of polyvinyl alcohol.

3. A polyvinyl alcohol-silver halide photographic emulsion adapted to give coatings resistant to the softening effect of hot rolls comprising silver halide dispersed in a polyvinyl alcohol protective colloid containing therein, a hardener prepared by heating together at IO-90 C. 1 parts of formaldehyde, 1 part of urea, and 1 part of polyvinyl alcohol in a proportion of 10 parts of hardener based on the formaldehyde therein per 100 parts of polyvinyl alcohol.

4. A method of preparing photographic products, the'emulsion layer of which is resistant to the softening effect of hot rolls, which comprises preparing a photographic emulsion of a silver halide dispersed in a polyvinyl alcohol protective colloid, adding thereto a hardener prepared by heating together at 50100 C. 0.5-1.5 parts of formaldehyde, 0.5-2 parts of urea, and 1-10 parts of polyvinyl alcohol, in a proportion of 1-27 parts of hardener based on the formaldehydetherein per 100 parts of polyvinyl alcohol present in the mixture, coating out the emulsion upon a support therefor and drying.

5. A method of preparing photographic paper, the emulsion coating of which is resistant to the softening effect of hot rolls, which comprises P g- 'z-p qt ra q' emu sion-v ess consisting of sily'er halidgv dispersed a polyvinyl alcoholprotective colloid, adding thereto a hardener prepared by heating-together at50-100" C. 05-15, parts; of formaldehyde, 0.5-2 partsof urea,-and 1 10. parts of p lyvinyl..alcohol in a proportion; of 1 -27 partsof hardener based on the formaldehyde therein per, 100 parts of polyvinyl alcohol, coating out the, emulsion as a thin layer upon a paper support therefor and drying the coating. I 1 U 6. A method of preparing a photographic product, the emulsion coating of which is resistant to the softening eifect of hot rolls, which comprises preparing a photographic emulsion essentially consisting of silver halide dispersed in a polyvinyl alcohol protective colloid, adding thereto a hardener prepared by heating together at YO- C. at a pH above 7, 1% parts of formaldehyde, lpart of urea, and 1 part of polyvinyl alcohol ina proportion .of:10 parts of hardener basedon the formaldehyde therein per 'parts of polyvinyl alcohol, coating out theemulsion upon a carrier thereforand drying 'the coating at substantially room temperature.

7. A method of preparing a photographic prod! uct, the emulsion coating ofwhich-is resistant to the softening effect of hotrolls, which comprises preparing ;a.;photographic emulsion of silver halide dispersed in a polyvinyl alcohol protective colloid, incorporating therein a small proportion of boric' acid, ands-adding thereto a hardener preparedbyheating togetherv at.,50 100 C. 0.545 parts-of formaldehyde,-0.52 parts of urea, and 110'parts of polyvinyl alcoholin a proportion of 1-27 parts: of hardener based on the formaldehyde therein per 100 parts of polyvinyl alcohol, coating out the photographic emulsion upon a support therefor,- setting the coating by fuming with gaseous 1 emmonia, andthen drying the coating. 1

WESLEY G. LOWE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 7 2,184,289 Dangelmajer Dec. 26, 1939 2,184,310 Meigset a1. Dec. 26, 1939 2,286,215 r LOWe June 16, 1942 2,376,371 Lowe et al May 22, 1945 V g FQREIGN PATENTS 7 Number 7 Country r v Date 361,910 Great Britain 'Nov. 16, 1931 

